1. Field of the Invention
The present invention is directed to a device used in an apparatus for crucible-free zone melting of crystalline rods of semiconductor material which device forms a container surrounding the lower end of the rod for receiving a supporting or stabilizing material.
2. Prior Art
In Wolfgang Keller U.S. Pat. application, Ser. No. 525,641, filed on Nov. 20, 1974 which issued as U.S. Pat. No. 3,923,468, a method and apparatus for supporting a lower end of a crystalline rod of semiconductor material during a floating zone melting process was disclosed. As pointed out in the application, the crystalline rod of semiconductor material was provided at its lower end with a seed crystal which was joined thereto by a melted junction point or surface. The rod with the crystal was mounted in a vertical position in the zone melting apparatus by mounting means attached to each end with the seed crystal at the lower end. The mounting means could be rotated and were moveable axially relative to an annular induction heating coil so that a melting zone could be progressively moved axially along the crystalline rod to form a single crystal of semiconductor material. In this application, the supporting device comprised an axially moveable funnel-shaped shell which was mounted on the lower mounting means for rotation therewith. The funnel-shaped shell could be axially shifted from a position axially below the junction surface between the rod and the seed crystal to a position surrounding the seed crystal, the cone-shaped or drawn end and the lower portion of the rod after the zone melting had progressed axially therefrom.
As mentioned in the Keller application for rods of a length exceeding approximately 50 cm, as the floating zone melting proceeded away from the junction a given distance which depends on the diameter of the rod, vibrations would occur which would effect the zone melting process. The funnel-shaped shell formed a container which surrounded the junction of the rod to the seed crystal and the cone-shaped or conical portion of the rod adjacent the junction which is formed for the purpose of avoiding crystal displacements in the crystal being formed. The funnel-shaped shell or container which surrounded the rod of semiconductor material was moved axially on the lower mounting means for the rod by an external drive mechanism. In the method described in the application, before the melting zone had reached the distance from the junction surface of the seed crystal and the crystalline rod of semiconductor material which was critical in terms of the application of vibrations, the funnel-shaped container was moved axially upward to such an extent that it surrounded the junction surface, and surrounded a conical region of the crystalline rod adjacent to the juntion surface and was then filled with the stabilizing or supporting medium.
A difficulty with the above structure was that it required a larger amount of axial space for allowing movement of the funnel-shaped container from a retracted position exposing the junction to allow the beginning of the zone melting to the position which surrounded the junction and the adjacent bottleneck or conical portion of the rod. Since the zone melting processes are conducted in an enclosed air-tight chamber which enables either the conducting of the process in a vacuum or within a controlled atmosphere, this additional axial space required for the shifting of the conical container either reduced the length of a rod that could be processed or in the case of processing long crystalline rods of semiconductor material, required an elongated gas-tight chamber for conducting the process.